1. Field of the Invention
The present invention relates to a semiconductor substrate and a method of manufacturing the same, and more specifically to a method of manufacturing a semiconductor substrate, by which the depth distribution of oxygen precipitate in a silicon (Si) wafer in particular can be controlled in order to reduce device failure and thereby to improve the production yield of the devices.
2. Description of the Prior Art
Conventionally, IG (intrinsic gettering) substrates in which BMD (bulk micro-defect) is formed inside the substrate to reduce device failure have been so far used. Here, the BMD implies oxygen precipitate. In the IG substrate, metallic impurities of the wafer surface can be eliminated by gettering these impurities with the use of the BMD, that is, by absorbing these impurities at the BMD located at the positions at which device characteristics are not directly influenced by the BMD. In this method, it is possible to reduce the device failure caused by the generation of crystalline defects and the increase of P-N junction leakage current due to contamination, for instance.
Here, since the gettering efficiency of the IG substrate can be increased in proportion to the BMD density, the high density of BMD in inner region of substrate is desirable.
In the conventional IG substrate, the BMD is formed in the interior of the substrate in accordance with the following method: the substrate is first heat treated at about 1200.degree. C. in an oxidizing atmosphere to form DZ (denuded zone) layer on the surface of the substrate as a non defective layer; and thereafter the substrate is further heat treated at a low temperature of about 800.degree. C. and at a medium temperature of about 1000.degree. C., respectively to form BMD only in the interior of the substrate.
In the conventional method, although it has been so far stated that there exist no defectiveness in the DZ layer, when the DZ layer is inspected minutely, it has been found that the DZ layer still includes BMD of a considerably high density.
FIG. 1A is a graphical representation showing the relationship between the BMD density and the depth from the surface of the conventional substrate, in which the solid line indicates the BMD density distribution of a CZ (Czochralski) substrate formed in a crystalline material grown in accordance with CZ method and the dashed line indicates the BMD density distribution of the IG substrate. In the case of the CZ substrate, the heat treatments at a lower temperature of about 800.degree. C. and a middle temperature of about 1000.degree. C. were carried out to visualize the BMD depth distribution. FIG. 1A indicates that even in the IG substrate there are many BMDs of about 10.sup.7 to 10.sup.8 pieces per cm.sup.3 even at a region of about 10 .mu.m depth from the surface which is to be a DZ layer of the substrate.
Further, FIG. 1B is the representation of BMD distribution obtained when the conventional IG substrate manufactured in accordance with a predetermined treatment is observed with a microscope on a cross section taken along the depth direction thereof, in which the reference numeral 31 denotes a substrate surface and 32 denotes BMDs. FIG. 1B indicates that a lot of BMDs exist in the region near surface.
In addition, it has been clarified that the BMD formed in the DZ layer (at which no defectiveness is required) cause defective failure.
As already explained, although in order to improve the gettering efficiency of the IG substrate, it is necessary to increase the BMD density, since the BMD density in the DZ layer is also increased with increasing BMD density in the interior of the substrate, there exists a limit in the practical improvement of the gettering capability. However, with the advance of the further microminiaturization of LSIs, the need of solving the above-mentioned problem has increased more and more.